This invention relates generally to microwave antennas and is disclosed particularly as utilized in a horn reflector antenna of the type disclosed in Dawson, U.S. Pat. No. 3,550,142. Such antennas have been in use for many years as microwave links in long distance telephone transmission and other data link communication service. Such antennas take the form of a conical section which is coupled at its narrow end to a microwave horn to serve as a conductive shield and wave guide of expanding area to an open end of the conical section which is adjacent to a reflector inclined at an angle to reflect microwave energy to a from the open end of the conical section as the antenna is used for two-way transmission and reception. Such antennas usually employ a sector of a paraboloid as the reflector, the focus of the paraboloid being at the apex of the conical section and forming the feed source for the horn which is coupled to the narrow end of the conical section. With the paraboloid reflector oriented at 45.degree. to the axis of the conical section, the antenna can be mounted with that axis vertical and have a horizontal aperture which is the size of the horizontal projection of the paraboloidal area.
In the prior art conical horn antennas have generally been in two forms, one in which the conical section is shaped with a rectangular cross-section and thus appears as an inverted, truncated pyramid with the paraboloidal reflector forming a roof over the wide end of the pyramid and the aperture being the opening from the top edge of the paraboloid to the top edge of the wall of the conical section. As such, the antenna forms a completely conductive enclosure except for the aperture opening which has been found to be advantageous both from the standpoint of emitting spurious radiation apart from the main beam and to avoid receiving unwanted incoming signals which are not on the main beam.
The Dawson patent referred to provides the modern configuration of the horn reflector antenna, and comprises as the physical enclosure a vertical right circular cone intersecting a horizontal right circular cylinder with a roof cap extending approximately 45.degree. above the intersection of the axes of the cone and cylinder to complete the conductive enclosure. As before, the aperture remains open and in the Dawson type antenna is the open end of the horizontal right circular cylinder. Suspended beneath the roof cap at an angle of 45.degree. is a paraboloidal sector reflector, the focus of the paraboloid being at the apex of the conical section and the horizontal projection of the paraboloid being the circular aperture at the open end of the right circular cylinder. This aperture is usually closed by a microwave transparent radome to permit the entire enclosure to be pressurized.
As mentioned in the Dawson patent, microwave absorber material has been used within the enclosure of such antenna for suppressing reflections. In the U.S. Pat. No. 3,936,837, to Coleman et al., the disclosure of lining the entire right circular cylinder with microwave absorber material is suggested and discloses the use of a corrugated conical feed for suppressing side lobe levels.
The U.S. Pat. No. 4,249,183, to Bui Hai, shows a parabolic dish antenna illuminating a planar reflector with a cylindrical wave guide between the antenna and reflector with the cylindrical wave guide lined with absorber material. European Patent Publication No. 0,000,305 discloses a horn fed reflector antenna with absorber material lining both the enclosing shield and the conical feed section.
The Assignee of the present application, Gabriel Electronics Incorporated of Scarborough, Me., has for many years sold the Dawson type antenna, with microwave absorber lining in both the circular cylinder and the conical section. In addition to suppressing unwanted reflections in the cylindrical portion of such antennas, microwave absorber linings have a marked effect on the side lobe pattern of such antennas. As the use of such antennas has increased and the placement of antennas operating on multi-microwave band assigned frequencies in close proximity to one another has occurred, the importance of side lobe levels has become more important, particularly the effort to reduce such side lobe levels to very low values relative to the main beam. As the efforts to achieve ever lower side lobe levels has continued, Gabriel has extended the absorber lining in the conical section progressively further down into narrower regions of the conical section. This expedient has reduced side lobe level and improved the overall radiation pattern envelope of such antennas, but as the microwave absorber has proceeded further down the conical section, its thickness relative to the diameter of the cone has become an important factor. As the thickness of microwave absorber (actually the double thickness, since the lining on opposite walls of the cone is involved) relative to the diameter of the conical section, the effective area of the paraboloidal reflector becomes shadowed by the presence of the microwave absorber, such that the gain of the antenna is reduced. Accordingly, the commercial antennas produced by Gabriel Electronics Incorporated in the past have been limited in the extent that microwave absorber could be extended further down into the narrow portion of the conical section by the consequent reduction in the gain of the antenna, as a design trade-off.
Recently a new commercial antenna has appeared on the market corresponding to the patent to Knop et al., U.S. Pat. No. 4,410,892. This Dawson type antenna has the extension of the cylindrical microwave absorber lining 22 a certain distance down into the conical section and then resorts to a different type of microwave absorber material 35 to extend the absorber lining further into the narrow diameter portions of the conical section, relying upon the thinness of the different absorber material 35 to avoid blocking or shadowing the reflector relative to the microwave source at the throat of the conical section.